1. Field of the Invention
This invention relates to gas-permeable membranes and their use in packaging, especially the packaging of fresh produce.
2. Introduction to the Invention
Fresh cut fruit and vegetables, and other respring biological materials, consume oxygen (O2) and produce carbon dioxide (CO2), at rates which depend upon temperature and the stage of their development. Their storage stability depends on the relative and absolute concentrations of O2 and CO2 in the atmosphere surrounding them, and on temperature. Ideally, a respiring material should be stored in a container whose permeability to O2 and CO2 is correlated with (i) the atmosphere outside the package, (ii) the rates at which the material consumes O2 and produces CO2, and (iii) the temperature, to produce and atmosphere within the container having O2 and CO2 concentrations equal to the optimum values for preservation of the material. The permeability to water vapor may also be significant. This is the principle behind the technology of controlled atmosphere packaging (CAP) and modified atmosphere packaging (MAP), as discussed, for example, in U.S. Pat. No. 4,734,324 (Hill), U.S. Pat. No. 4,830,863 (Jones), U.S. Pat. No. 4,842,875 (Anderson), U.S. Pat. No. 4,879,078 (Antoon), U.S. Pat. No. 4,910,032 (Antoon), U.S. Pat. No. 4,923,703 (Antoon), U.S. Pat. No. 5,045,331 (Antoon), U.S. Pat. No. 5,160,768 (Antoon) and U.S. Pat. No. 5,254,354 (Stewart), and European Patent Applications Nos. 0,351,115 and 0,351,116 (Courtaulds). The disclosures of each of these publictions is incorporated herein by reference.
The preferred packaging atmosphere depends on the stored material. For example, some materials, e.g. broccoli, are best stored in an atmosphere containing 1-2% O2 and 5-10% CO2. For other materials, an atmosphere containing 1-2% O2 and 12-30% CO2, e.g. about 15% CO2, is preferred. Thus, CO2 concentrations of 10 to 30% slow the respiration rate of some fruit and rduce the activity of some decay-causing organisms; for example, a CO2 concentration of 20% delays grey mold decay in rasberries and extends their shelf life.
Although much research has been crried out, known packaging techniques have many shortcomings for respiring biological materials. We have discovered, in accordance with this invention, that by forming thin polymeric coatings on microporous films, it is possible to create gas-permeable membranes which have novel and desirable combinations of O2 permeabilikty, change in O2 permeability with temperature, and ratio of CO2 permeability to O2 permeability. Improved results can be obtained using a wide range of microporous base flims and coating polymers. However, a particular advantage of the present invention is that it makes it possible to design packages which are tailored to the requirements of particular respiring materials. As further discussed below, the gas-permeable membranes of this invention are generally used as control sections which provide the sole, or at least the principal, pathway for gases to enter or leave a sealed container containing a respiring material.
In describing the invention below, the following abberviations, definitions, and methods of meassurement are used. OTR is O2 permeability. COTR CO2 permeability. OTR and COTR values are given in ml/m2.atm.24 hrs, with the equivalent in cc/100 inch2.atm.24 hrs given in parentheses. OTR and COTR were measured using a permeability cell (supplied by Millipore) in which a mixture of O2, CO2 and helium is applied to the sample, using a pressure of 0.7 kg/cm2 (10 psi) except where otherwise noted, and the gases passing through the sample were analyzed for O2 and CO2 by a gas chromatograph. The cell could be placed in a water bath to control the temperature. The abbreviation P10 is used to denote the ration of the oxygen permeability at a first temperaturre T1xc2x0 C. to teh oxygen permeablility at a second temperature T2, where T2 is (T1xe2x88x9210)xc2x0 C., T1 being 10xc2x0 C. and T2 being 0xc2x0 C. unless otherwise noted. The abbreviation R is used to denote the ratio of CO2 permeability to O2 permeability, both permeabilities being measured at 20xc2x0 C. unless otherwise noted. Pore sizes given in this specification are measured by mercury porosimetry or an equivalent procedure. Parts and percentages are by weight, temperatures are in degrees Centigrade, and molecular weights are weight average molecular weights expressed in Daltons. For crystalline polymers, the abbreviation To is used to denote the onset of melting, the abbreviation Tp is used to denote the crystalline melting point, and the abbreviation xcex94H is used to denote the heat of fusion. To, Tp and xcex94H are measured by means of a differential scanning calorimeter (DSC) at a rate of 10xc2x0 C./minute and on the second heating cycle. To and Tp are measured in the conventional way well known to those skilled in the art. Thus Tp is the temperature at the peak of the DSC curve, and To is the temperature at the intersection of the baseline of the DSC peak and the onset line, the onset line being defined as the tangent to the steepest part of the DSC curve below Tp.
Typically, a microporous film has an R ratio of about 1, and OTR and COTR values which (i) are very high, (ii) do not change much with the thickness of the film, and (iii) do not change much with temperature (leading to P10 ratios of about 1). A continuous polymeric layer, on the other hand, typically has an R ratio substantially greater than 1 (generally 2 to 6, depending on the polymer itself), and has OTR and COTR values which (i) are relatively low, (ii) are inversely proportional to the thickness of the layer, and (iii) change substantially with temperature (leading to P10 ratios substantially graeter than 1, generally at least 1.3). At practical thicknesses, such continuous polymeric layers have OTR and COTR values which are undesirably low.
We have discovered that when a membrane is prepared by coating a thin layer of a polymer onto a suitable microporous film, it has permeability characteristics which depend on both the coating polymer and the microporous film. We do not know exactly why this is so, and theresults achieved by this invention do not depend upon any theory of its operation. However, we believe that the coating polymer effectively blocks most, but not all, of the pores of the microporous film (with the smaller pores being preferentially blocked); and that as a result, the permeability of the membrane results in part from gases which pass through the unblocked pores and in part from gases which pass through the coating polymer. In any event, the invention makes it possible to prepare novel membranes having very desirable permeability characteristics, and to achieve controlled variation of those characteristics. For example, the invention makes it possible to prepare membranes having an OTR greater than 775,000 (50,000), e.g. 1,550,000 (100,000) to 3,875,000 (250,000), or even higher, e.g. up to 7,750,000 (500,000) or mnore, a P10 ratio of at least 1.3, e.g. at least 2.6, and an R ratio of at least 1.5, e.g. at least 2.0.
The microporous film and the coating polymer must be selected and correlated to produce a membrane having particular properties, but those skilled in the art will have no difficulty, having regard to the disclosure in this specification and their owin knowledge, in achieving a wide range of usefuil results.
The size and distribution of the pores in the microporous film are important factors. If the pores are too small, the coating polymer tends to form a continuous layer which is either too thin to be durable under routine handling, or too thick to have an acceptable OTR. If the pores are too large, the coating polymer may be unable to bridge over them, so that the coating polymer plays little or no part in determining the permeability characteristics of the membrane. This may happen even if the average pore size is relatively low, if the pores have a wide range of sizes; for example the coating polymer may effectively block many of the pores, but still fail to block the larger pores, whose permeability then dominates the permeability of the membrane as a whole.
The roughness of the microporous film can also be an important factor. The coating weight of the coating polymer must be very small, and In consequence the thickness of the coating polymer is also very small. If such a thin layer is in intimate contact with an irregular surface, it is more likely to be able to withstand abrasive forces during use than a layer of the same thickness which lies on a relatively smooth surface.
The coating polymer should be selected so that the membrane has a desired P10 ratio and a desired R ratio, and should be coated onto the microporous film at a coating weight which results in a membrane having the desired balance betwecn the permeability characteristics of the microporous film and of the coating polymer. For example, by choosing a crystalline coating polymer whose Tp is within or a little below an expected range of storage temperatures, it is possible to produce a membrane whose P10 is relatively large in the storage temperature range; furthermore, the size of the P10 ratio can be increased by increasing the xcex94H of the coating polymer. Similarly, a membrane having a relatively large (or small) R ratio can be produced by selecting a coating polymer having a relatively high (or small) inherent R ratio. In this way, the invention makes it possible to produce membranes whose properties can be tailored, much more closely than was previously possible, to the needs of a particular respiring hinlogical material.
In a first preferred aspect, this invention provides a gas-permeable membrane which comprises
(a) a microporous polymeric fllm, and
(b) a pulymeric coating on the microporous film, the polymeric coating changing the permeability of the microporous film so that the membrane
(i) has a P10 ratio, over at least one 10xc2x0 C. range between xe2x88x925 and 15xc2x0 C. of at least 1.3;
(ii) has an oxygen permeability (OTR), at all temperatures between 20xc2x0 and 25xc2x0 C., of at least 775,000 ml/m2.atm.24 hrs (50,000 cc/100 inch2.atm.24 hrs); and
(iii) has an R ratio of at least 1.5;
the P10, OTR and R values being measured at a pressure of 0.035 kg/cm2.
In a second preferred aspect, this invention provides a package which is stored in air and which comprises
(a) a sealed container, and
(b) within the sealed container, a respiring biological material and a packaging atmosphere around the biological material;
the scaled container including one or more permeable control sections which provide substantially the only pathways for oxygen and carbon dioxide to enter or leavc the packaging atmosphere, at least one said permeable control section being a gas-permeable membrane as defined in the first aspect of the invention